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At 490g, this is not something that most backpackers will want to carry on multi-day trips, but it has some unique features that make it superb at what it does. The purifier contains a large hollowfibre filter with a pore size of 0.02 microns, which means that it will filter out viruses. Instead of squeezing, there's a large handle that is used to pump water through the filter. This makes it easy to get an impressive flow rate going - MSR claims 2.5 litres per minute, and I'd say that's a conservative estimate. You don't need to faff with filling a bladder with dirty water for filtering, either. Just drop the hose into any water source and get pumping! The end of the hose is protected by afine piece of gauze to prevent debris from entering the filter. The Guardian can screw onto the top of a Nalgene bottle or an MSR DromLite water bag (I tested it with this). The design is extremely durable; MSR says it will withstand a 6ft drop onto concrete, and unlike every other hollowfibre filter tested it can be stored in freezing temperatures without damage. This is a major advantage. It's a self-cleaning design and will keep going with minimal maintenance, lasting for over 10,000 litres. Although expensive and heavy, this is the best product tested for actually making water safe to drink. For serious expedition use to remote areas, especially if you're in a group, there's nothing more effective.